The present invention is related to stereo-specific synthesis and more particularly to a method of preselecting S or R stereo-isomerism of shikimic acid derivatives and a process for producing these compounds from readily available low-cost starting materials.
The present invention relates to a multi-step process for the preparation of 4,5-diamino shikimic acid derivatives, especially for the preparation of (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylic acid ethylester and its pharmaceutically acceptable addition salts starting from isophthalic acid derivatives, individual process steps thereof, as well as new specific intermediates.
4,5-diamino shikimic acid derivatives, especially the (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylic acid ethylester and its pharmaceutically acceptable addition salts are potent inhibitors of viral neuraminidase (J. C. Rohloff et al., J. Org. Chem., 1998, 63, 4545-4550; WO 98/07685).
A multi step synthesis of (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylic acid ethylester from (xe2x88x92)-quinic acid or (xe2x88x92)-shikimic acid is described in (J. C. Rohloff et al, loc.cit.).
Both (xe2x88x92)-quinic acid and (xe2x88x92)-shikimic acid are starting compounds which are rather expensive and hardly accessible in technical quantities. A multi-step synthesis capable to run on a technical scale should therefore preferably be based on starting compounds which are more attractive in price and available in technical quantities.
A process of the present invention for preparing a 4,5 di amino shikimic acid derivitive having formula 1a 
wherein R1 is an optionally substituted alkyl group, R2 is an alkyl group and R3 and R4, independently of each other are H or a substituent of an amino group, with the proviso that both R3 and R4 are not H includes hydrogenating an isophthalic acid derivative of formula II; 
wherein R5is H or lower alkyl thereby forming an all-cis-cyclohexane dicarboxylate of formula III; 
then selecting a stereo-selective hydrolysis and dealkylation sequence from the group consisting of: a) in the case when R5=H, stereo-selectively hydrolyzing the all-cis-cylohexane dicarboxylate of formula (III), thus forming the (S)- or (R)-cyclohexane monoacid of formula IVa or IVb, b) in the case where R5=lower alkyl, stereo-selectively hydrolyzing the alkoxy all-cis-cyclohexane dicarboxylate of formula (III), dealkylating to form the (S)- or (R)-cyclohexane monoacid of formula IVa or IVb and c) in the case where R5=lower alkyl, dealkylating the alkoxy all-cis-cyclohexane dicarboxylate of formula (III) and then stereo-selectively hydrolyzing the all-cis-cyclohexane dicarboxylate of formula (III) to form the (S)- or (R)-cyclohexane mono acid of the formulae IVa or IVb; 
converting the cyclohexane monoacid of formula (IVa) to an oxazolidinone of the formula Va; 
transforming the oxazolidinone (Va) into a cyclohexenol (VIa) 
wherein R6 is an amino protecting group; converting cyclohexenol (VIa) to an azide VIIa; and 
reducing and acylating azide (VIIa); forming the respective acylated amine (VIIIa) 
thereby forming the 4,5-diamino shikimic acid derivative (Ia) by removing the amino protecting group R6. In performing the series of steps of the process of the invention, the steps forming compounds from IVa are exemplary of similar steps forming a similar series of compounds from IVb to VIIIb. This alternate series of steps are equally preferred.
The stereo-selective synthethic method of the invention allows the use of lower-cost more accessible starting materials, provides the practitioner the ability to preselect the desired stereochemistry at several chiral centers on the molecule by selecting either an esterase or a lipase, thereby greatly improving the efficiency and accessiblity to shikimic acid derivatives that are known to be potent inhibitors of viral neuraminidase.
The preferred embodiments of the invention are described herein in detail and should be considered exemplary, not limitive. The scope of the invention is measured by the appended claims and their equivalents. The present invention relates to a process for the preparation of a 4,5-diamino shikimic acid derivatives of formula Ia 
and pharmaceutically acceptable addition salts thereof
wherein R1 is an optionally substituted alkyl group,
R2 is an alkyl group and
R3 and R4, independent of each other are H or a substituent of an amino group, with the proviso that both R3 and R4 are not H
and which is characterized in that
in step a)
an isophthalic acid derivative of the formula 
wherein R1 and R2 are as above and R5 is H or lower alkyl
is hydrogenated to form an all-cis-cyclohexane dicarboxylate of the formula 
wherein R1, R2 and R5 are as above,
in step b)
the cyclohexane dicarboxylate of formula (III) is, if R5=H, stereo-selectively hydrolyzed to form the (S)- or (R)-cyclohexane monoacid of formulas IVa or IVb or, if R5=lower alkyl, either dealkylated first and then stereo-selectively hydrolyzed or stereo-selectively hydrolyzed first and then dealkylated to form the (S)- or (R)-cyclohexane mono acid of the formula 
wherein R1 and R2 are as above,
in step c)
the cyclohexane monoacid of the formula (IVa) is further converted to an oxazolidinone of the formula 
wherein R1 and R2 are as above,
in step d)
the oxazolidinone of formula (V) is transformed into a cyclohexenol of the formula 
wherein R1 and R2 are as above and R6is an amino protecting group
in step e)
the cyclohexenol of formula (VI) is further converted into an azide of formula 
wherein R1, R2 and R6 are as above,
in step f)
the azide of formula (VII) is reduced and acylated to form the acylated amine of the formula 
wherein R1, R2, R3, R4 and R6 are as above, and in step g)
the acylated amine of the formula (VIII) is finally transferred into the 4,5-diamino shikimic acid derivative of formula (I) by removing the amino protecting group R6 and if necessary by forming the respective pharmaceutically acceptable salt.
The term alkyl in R1 has the meaning of a straight chain or branched alkyl group of 1 to 20 C-atoms, expediently of 1 to 12 C-atoms. Examples of such alkyl groups are methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, tert-butyl, pentyl and its isomers, hexyl and its isomers, heptyl and its isomers, octyl and its isomers, nonyl and its isomers, decyl and its isomers, undecyl and its isomers and dodecyl and its isomers. This alkyl group can be substituted with one or more substituents as defined in e.g. WO 98/07685. Suitable substituents are alkyl of 1 to 6 C-atoms (as defined above), alkenyl of 2 to 6 C-atoms, cycloalkyl with 3 to 6 C-atoms, hydroxy, alkoxy with 1 to 6 C-atoms, alkoxycarbonyl with 1 to 6 C-atoms, F, Cl, Br, and J.
Preferred meaning for R1 is 1-ethylpropyl.
R2 is a straight chain or branched alkyl group of 1 to 12 C-atoms, expediently of 1 to 6 C-atoms as exemplified above.
Preferred meaning for R2 is ethyl.
R5 is a lower n-alkyl group of 1 to 3 C-atoms, preferably methyl.
R3 and R4 are substituents of an amino group used and known in the art and described e.g. in WO 98/07685.
R3 and R4 preferably stand for alkanoyl groups, more preferably lower alkanoyl with 1 to 6 C-atoms such as hexanoyl, pentanoyl, butanoyl (butyryl), propanoyl (propionyl), ethanoyl (acetyl) and methanoyl (formyl). Preferred alkanoyl group and therefore preferred meaning for R3 is acetyl and for R4is H.
R6 is a common amino protecting group used and known in the art and described e.g. in xe2x80x9cProtective Groups in Organic Chemistryxe2x80x9d, Theodora W. Greene et al., John Wiley and Sons Inc., New York, 1991, 315-385.
R6 suitably is benzyloxycarbonyl (Z), tert-butyloxycarbonyl (BOC), allyloxycarbonyl (AllOC) or 9-fluorenylmethoxycarbonyl (FMOC), preferably tert-butoxycarbonyl (BOC).
Preferred 4,5-diamino shikimic acid derivative of formula (I) is the (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylic acid ethylester and the (3R,4R,5S)-4-acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylic acid ethyl ester phosphate (1:1).
The process of the present invention from step c) onwards run with the cyclohexane monoacid of the formula (IVb) retains the stereo-specificity introduced in the stereo-selective hydrolysis of the cyclohexane dicarboxylate and produces the (+)-enantiomer of the 4,5-diamino shikimic acid derivative having the formula 
wherein R1, R2, R3 and R4 are as above and all the (+)-enantiomers of the corresponding intermediates.
Step a)
Step a) comprises the hydrogenation of an isophthalic acid derivative of the formula (II) to an all-cis-cyclohexane dicarboxylate of the formula (III).
Hydrogenation takes place with hydrogen in the presence of a common hydrogenation catalyst which may be applied on an inert support. Suitable hydrogenation catalysts are rhodium or ruthenium applied in an amount of 1 to 10% on an inert support, such as on aluminum oxide or charcoal. The hydrogenation can be effected in an inert solvent like ethylacetate, ethanol, tetrahydrofuran or tert-butyl methyl ether at temperatures between 20xc2x0 C. and 150xc2x0 C. and at hydrogen pressures between 1 bar and 200 bar.
The resulting cyclohexane dicarboxylate of the formula (III) shows an all-cis meso form and therefore is optically inactive.
Step b)
Step b) comprises a stereo-selective enzymatic hydrolysis and, if necessary, a dealkylation of the cyclohexane dicarboxylate of the formula (III) to either the (S)- or (R)-cyclohexane mono acid of the formulas (IVa) or (IVb).
Starting from the cyclohexane dicarboxylate of formula (III) with R5=H stereo-selective enzymatic hydrolysis can directly take place, however, starting from the cyclohexane dicarboxylate of formula (III) with R5=lower alkyl dealkylation can either take place before or after the stereo-selective hydrolysis.
The dealkylation, can take place with an alkali iodide in the presence of a trialkylhalogen silane. Dealkylation preferably is a demethylation and preferably sodium iodide together with trimethylchlorosilane is used. This dealkylation as a rule is performed in an inert solvent, such as in acetonitrile at temperatures between 20xc2x0 C. and 80xc2x0 C.
Stereo-selective hydrolysis comprises an enzymatic hydrolysis of the cyclohexane dicarboxylate of the formula (III), whereby the choice of the enzyme determines whether the (S)-monoacid of the formula 
or the (R)-monoacid isomer of the formula 
can be obtained.
In order to achieve the 4,5-diamino shikimic acid of formula (Ia) with the desired stereo configuration the subsequent reaction steps are performed with the (S)-monoacid of formula (IVa).
Starting with the all-cis-cyclohexane dicarboxylate of formula III with R5=H suitable enzymes to gain the (S)-isomer of formula (IVa) are esterases of the EC class 3.1.1.1, preferably mammalian esterases (e.g. from pig, bovine or horse). The most preferred enzyme is pig liver esterase (which is subsequently termed PLE). Commercial preparations of PLE can be purchased e.g. from Roche Diagnostics, Fluka, Sigma, Amano or Altus. Also less purified PLE preparations (e.g. xe2x80x98PLE technical gradexe2x80x99 from Roche Diagnostics) or only poorly purified preparations (e.g. such as xe2x80x98pig liver acetone powderxe2x80x99 from Fluka) can be used as well as PLE preparations with enriched or separated isozyme fractions (like e.g. Chirazyme E-1 or Chirazyme E-2 from Roche Diagnostics). As an alternative the enzymes may be used in immobilized form.
The substrate is applied as a suspension in an aqueous solution in a 5-15% concentration (w/w), preferably around 10%. A suitable reaction temperature is room temperature to 35xc2x0 C., a suitable reaction pH between 6.5 and 8.5.
As to the aqueous phase, common buffer solutions known to be used for biochemical conversions are used like e.g. phosphate or Tris-buffer in a concentration of 5-50 mM. Such a buffer solution can additionally contain a salt like e.g. NaCl or KCl in a concentration of 50 to 300 mM. A preferred buffering system contains 0.1 M KCl and 10 mM Tris-hydrochloride pH 8.0.
After addition of the enzyme the pH of the reaction mixture at the selected value is maintained under stirring by the controlled addition of a base such as NaOH or KOH, whereby the formed monoacid goes into solution and the reaction mixture becomes rather clear.
After termination of the reaction, the product is worked up by acidification of the reaction mixture and extraction with a common organic solvent.
Starting with the all-cis cyclohexane dicarboxylate of formula III with R5=H or lower alkyl, preferably methyl, suitable enzymes to gain the (R)-isomer of formula (IVb) are lipases of the EC class 3.1.1.3. Suitable representatives of this class are the lipases from Aspergillus oryzae (commercially available at Fluka), Thermomyces lanuginosa (formerly termed Humicola lanuginosa; e.g. from Novo Nordisk) and from Mucor miehei (e.g. from Novo Nordisk). Again, also less purified crude enzyme preparations may be used.
Again, as an alternative, the preselected enzymes may be used in immobilized form. The reaction is carried out in an aqueous or an aqueous-organic biphasic system. Preferred is a biphasic system with a water-immiscible apolar solvent as co-solvent. Suitable co-solvents are alkanes or cycloalkanes, preferred is cyclohexane.
The substrate is applied (as a suspension) in the mono- or biphasic system in 5-10% overall concentration (w/w). A suitable reaction temperature is room temperature to 35xc2x0 C., a suitable reaction pH between 6.5 and 8.5.
As to the aqueous phase, common buffer solutions known to be used for biochemical conversions are used like e.g. phosphate, borate or Tris-buffer in a concentration of 5-50 mM. Such a buffer solution can additionally contain a salt like e.g. NaCl, KCl or a polyhydric alcohol such as a sugar (e.g. glucose) in a concentration of 50 to 300 mM. A preferred buffering system could e.g. contain 0.1 M glucose and 5 mM sodium phosphate pH 7.0. The ratio organic solvent/aqueous phase is in the range of 1:10 to 1:1.
After addition of the enzyme the pH of the reaction mixture is maintained under stirring at the selected value by the controlled addition of a base such as NaOH or KOH.
After termination of the reaction, the product is worked up by acidification of the reaction mixture and extraction with a common organic solvent.
Step c)
Step c) comprises the conversion of the cyclohexane mono acid of the formula (IVa) into the oxazolidinone of formula (V).
This conversion can take place applying the principles of a Curtius or of a Hoffmann degradation. Where in the Hoffmann degradation the oxazolidinone is formed by transformation of the cyclohexane monoacid into the respective cyclohexane monoamide and by subsequent ring formation e.g. with a hypochlorite, the Curtius degradation involves the formation of the cyclohexane azide intermediate.
As a suitable variation of the Curtius degradation a Yamada-Curtius degradation using dialkylphosphorylazides or diarylphosphoryl azides, preferably diarylphosphoryl azides, most preferably diphenyl phosphoryl azide (DPPA) can be applied.
The Yamada-Curtius degradation takes place in the presence of a tertiary amine, preferably triethylamine and in an inert solvent such as e.g. methylene chloride or ethylacetate.
Step d)
Step d) covers the transformation of the oxazolidinone of formula (V) into a cyclohexenol of formula (VI).
This transformation comprises the introduction of an amino protecting group R6 and a subsequent base induced transformation to the cyclohexenol of formula (VI).
Suitable substituents of the amino group R6 are as stated above, however, the BOC group is the preferred group. Introduction of the amino protecting group is known to the skilled in the art.
Suitable base for the subsequent base induced transformation is an alkali-hydride, an alkali-alcoholate, diazabicyclo undecen (DBU) or a tetraalkyl guanidine. Preferred base is sodium hydride applied in amounts of 0.5 to 25 mol %.
Usually the reaction takes place in an inert solvent such as methylene chloride, toluene, tetrahydrofuran, ethyl acetate at reflux temperature of the respective solvent.
The cyclohexenol of formula (VI) can be isolated from the reaction mixture by methods known to the skilled in the art.
Step e)
Step e) comprises the formation of an azide of formula (VII).
This step involves in a first sequence, the transformation of the hydroxy group into a suitable leaving group and in a second sequence, the azide formation, thereby leading to an inversion of configuration at the reaction center.
The transformation of the OH group into a leaving group can be performed by sulfonylation, i.e., converting the OH group into a sulfonic acid ester.
Agents commonly used for producing such sulfonic esters are e.g. the halogenides or the anhydrides of the following sulfonic acids: methane sulfonic acid, p-toluenesulfonic acid a p-nitrobenzenesulfonic acid, p-bromobenzenesulfonic acid or trifluoromethanesulfonic acid.
Preferred agent is a halogenide or anhydride of trifluoro methane sulfonic acid such as trifluoro methane sulfonic anhydride.
The sulfonylating agent is expediently added in an amount of 1.0 to 1.5 equivalents relating to one equivalent of the cyclohexenol of formula VI in presence of about two equivalents of a suitable base.
The reaction usually takes place in an inert solvent such as in methylene chloride and at reaction temperatures between xe2x88x9220xc2x0 C. and room temperature.
The sulfonic acid ester formed can be isolated and purified, e.g. by crystallization or directly be introduced into the following reaction sequence.
Azide formation is effected by treating the sulfonic acid ester intermediate previously obtained with a suitable azide whereby inversion of the configuration takes place. Azides commonly used are alkaliazides like sodium azide in amounts of 1 to 2 equivalents.
The reaction takes place in a solvent such as in dimethyl sulfoxide, N,N-dimethylformamide, ethanol or acetone at temperatures between xe2x88x9210xc2x0 C. and 50xc2x0 C.
Step f)
Step f) covers the reduction of the azide and the subsequent acylation of the resulting amine to form the respective acylated amine of the formula (VIII).
Reduction takes place either by a) a classical metal catalysed hydrogenation with hydrogen or b) by reduction of the azide with a phosphine.
According to method a) common hydrogenation catalysts such as e.g. Pd, Pt, Raney-Ni or Raney-Co catalysts which may be applied on an inert support can be used.
The hydrogenation can take place in a suitable organic solvent e.g. in ethylacetate at temperatures between 20xc2x0 C. and 60xc2x0 C. at at hydrogen pressures between 1 and 50 bar.
Phosphines which according to method b) can suitably be used are trioctyl phosphine, triisobutyl phosphine and tri-n-butyl phosphine. Most preferred phosphine is the tri-n-butyl phosphine.
Typically the reduction is performed in a polar solvent such as in ethylacetate or in tetrahydrofuran in presence of 1 to 20 equivalents of water. The reaction temperature, depending on the phosphine used, as a rule is chosen in the range of xe2x88x9220xc2x0 C. and 50xc2x0 C. The amine formed can be isolated but is preferably directly acylated in the following reaction sequence.
Acylation can be effected using acylating agents in the presence of a base and at conditions known to the skilled in the art. Suitable acylating agents as a rule are aliphatic or aromatic carboxylic acid halides or anhydrides. Preferred acylating agents are the acetylating agents such as acetyl chloride or acetanhydride.
Step g)
Step g) comprises the removal of the amino protecting group R6 and, if necessary, the formation of the respective pharmaceutically acceptable salt of the 4,5-diamino shikimic acid derivative of formula (I).
The amino protecting group R6 can be removed following methods well known to the skilled in the art . The preferred BOC group can e.g. easily be split off with HBr in acetic acid at room temperature or with HCl in ethylacetate. The free amine can then be liberated with e.g. an aqueous base and then further be transformed into the pharmaceutically acceptable addition salt following the methods described in J. C. Rohloff et al., J. Org. Chem. 63, 1998, 4545-4550; WO 98/07685).
The term xe2x80x9cpharmaceutically acceptable acid addition saltsxe2x80x9d embraces salts with inorganic and organic acids, such as hydrochloric acid, hydrobromic acid, nitric acid, sulfuric acid, phosphoric acid, citric acid, formic acid, fumaric acid, maleic acid, acetic acid, succinic acid, tartaric acid, methane sulfonic acid, p-toluenesulfonic acid and the like.
The salt formation is effected at room temperature in accordance with methods which are known per se and which are familiar to any person skilled in the art.
The preferred pharmaceutically acceptable acid addition salt is the 1:1 salt with phosphoric acid which can be formed preferably in ethanolic solution at a temperature of xe2x88x9220xc2x0 C. to 50xc2x0 C.
The invention further comprises a process for the preparation of an isophthalic acid derivative of the formula 
wherein R1, R2 and R5 are as above
which is characterized in that a dialkoxyphenol of the formula 
wherein R5 is as above
is
in step aa)
converted into a trialkoxybenzene of the formula 
wherein R1 and R5 are as above,
and in step ab) further halogenated to a dihalotrialkoxy benzene of the formula 
wherein R1 and R5 are as above and X stands for a halogen atom and finally
in step ac)
carbonylated to form the product of the formula II.
Step aa)
Etherification of the dialkyloxyphenol of formula X can be performed with the methane sulfonic acid ester of the respective alcohol R1OH. Reaction as a rule takes place in the presence of a strong base such as an alkali alcoholate in an inert solvent.
Alternatively etherification can happen under Mitsunobu conditions (O. Mitsunobu, Synthesis, 1981, 1) i.e. by treatment of the dialkoxyphenol of formula X with the respective alcohol R1OH in the presence of diisopropyl azodicarboxylate (DIAD) and triphenyl phosphine in a suitable inert solvent like tetrahydrofuran.
Step ab)
The halogenation in this step preferably is a dibromination.
A suitable bromination agent for the trialkoxybenzene of formula XI is e.g. N-bromo succinimide (NBS).
Dibromination usually takes place with 2 equivalents NBS in a polar solvent such as in N,N-dimethylformamide at temperatures between xe2x88x9210xc2x0 C. and 50xc2x0 C.
Step ac)
Carbonylation of the dihalotrialkoxybenzene of formula XII can be performed with carbon monoxide in the presence of suitable catalyst in an alcoholic solvent like ethanol.
Suitable catalysts are metal complexes formed by a metal compound of the group VIII element of the periodic table and a phosphine compound e.g. of palladium acetate and 1,3-bis (diphenyl phosphino) propane (dppp) or triphenylphosphine.
Usually the reaction is performed at temperatures of 80xc2x0 C. to 150xc2x0 C. and at CO-pressures up to 20 bar.
The invention further comprises a process for the preparation of an all-cis-cyclohexane dicarboxylate derivative of the formula 
wherein R1, R2 and R5 are as above
which is characterized in that an isophthalic acid derivative of the formula 
wherein R1, R2 and R5 are as above is hydrogenated.
This step is identical to step a) of the multi-step synthesis described herein above. The respective description of step a) is incorporated herein by reference.
The invention further comprises a stereo-selective hydrolysis and, if necessary, a dealkylation of an all-cis-cyclohexane dicarboxylate of the formula 
wherein R1, R2 and R5 are as above,
to form the (S)- or (R)-cyclohexane monoacid of the formulas. 
wherein R1 and R2 are as above.
This step is equivalent to step b) of the multi-step synthesis described herein above. The respective description of step b) is incorporated herein by reference.
The following key intermediates are new and not known to the state of the art, they accordingly are an essential element of the present invention. 
wherein R1, R2 and R5 are as above, preferably 5-(1-ethyl-propoxy)-4,6-dimethoxy isophthalic acid ethyl ester with R1=1-ethyl-propyl, R2=ethyl and R5=methyl. 
wherein R1, R2 and R5 are as above, preferably all-cis-5-(1-ethyl-propoxy)-4,6-dimethoxy-cyclohexane-1,3-dicarboxylic acid diethylester with R1=1-ethyl propyl, R2=ethyl and R5=methyl and all-cis-5-(1-ethyl-propoxy)-4,6-dihydroxy-cyclohexane-1,3-dicarboxylic acid diethylester with R1=1-ethyl propyl, R2=ethyl and R5=H. 
wherein R1 and R2 are as above, preferably all-cis-(1R,3S,4S,5S,6R)-5-(1-ethyl propoxy)-4,6-dihydroxy cyclohexane-1,3-dicarboxylic acid 1-ethyl ester with R1=1-ethyl propyl, R2=ethyl 
wherein R1 and R2 are as above, preferably all-cis-(1S,3R,4R,5R,6S)-5-(1-ethyl propxy)-4,6-dihydroxy cyclohexane-1,3-dicarboxylic acid 1-ethyl ester with R1=1-ethyl propyl, R2=ethyl. 
wherein R1 and R2 are as above, preferably (3aS,5R,6R,7R,7aS)-7-(1-ethyl propoxy)-6-hydroxy-2-oxo-octahydrobenzooxazole-5-carboxylic acid ethyl ester with R1=1-ethyl propyl and R2=ethyl and (3aR,5S,6S,7S,7aR)-7-(1-ethyl propoxy)-6-hydroxy-2-oxo-octahydrobenzooxazole-5-carboxylic acid ethyl ester with R1=1-ethyl propyl and R2=ethyl. 
wherein R1, R2 and R6 are as above, preferably (3R,4S,5S)-5-tert.-butoxy carbonyl-amino-3-(1-ethyl-propoxy)-4-hydroxy cyclohex-1-ene carboxylic acid ethyl ester (VIa) with R1=1-ethyl propyl, R1=ethyl and R6=tert-butoxy carbonyl and (3S,4R,5R)-5-tert-butoxy carbonyl-amino-3-(1-ethyl-propoxy)-4-hydroxy cyclohex-1-ene carboxylic acid ethyl ester (VIb) with R1=1-ethyl propyl, R2=ethyl and R6=tert-butoxy carbonyl. 
wherein R1, R2and R6 are as above, preferably (3R,4R,5S)-4-azido-5-tert.-butoxy carbonylamino-3-(1-ethyl propoxy) cyclohex-1-ene carboxylic acid ethyl ester (VIIa) with R1=1-ethyl propyl, R2=ethyl and R6=tert-butoxy carbonyl and (3S,4S,5R)-4-azido-5-tert.-butoxy carbonylamino-3-(1-ethyl propoxy) cyclohex-1-ene carboxylic acid ethyl ester (VIIb) with R1=1-ethyl propyl, R2=ethyl and R6=tert-butoxy carbonyl
The following examples shall illustrate the invention in more detail without limiting it.